Permeable cement composition and method for preparing the same

ABSTRACT

An improved permeable cement composition for formation of downhole sand screens. The improved composition includes an effective amount of a surfactant. Addition of the surfactant to the permeable cement composition yields at least a pumpable slurry with a minimal amount of water or brine solution.

BACKGROUND OF THE INVENTION

The current invention provides an improved permeable cement composition suitable for use in the downhole environment. The improved permeable cement composition is suitable for use in a wide array of downhole applications including but not limited to formation of sand screens and hydraulic fracturing.

The improved cement will be useful in overcoming problems associated with the production of fluids from unconsolidated or loosely consolidated subterranean formations. Fluids produced from subterranean formations of this nature typically carry sand into the well bore. The presence of the sand in the produced fluids rapidly erodes metal tubular goods and other production equipment. Eventually, the equipment must be replaced substantially increasing the costs of operating the wells.

Heretofore, gravel packs have been utilized in wells to prevent the production of formation sand. In gravel packing operations, a pack of gravel, e.g., graded sand, is placed in the annulus between a screen or a perforated or slotted liner and the walls of the well bore in the producing interval. The resulting structure filters sand from produced fluids.

Gravel packs successfully prevent the production of sand with formation fluids; however, they often fail and require replacement. While the initial installation of a gravel pack adds considerable expense to the cost of completing a well, the subsequent removal and replacement of a failed gravel pack adds an even greater financial burden to the operation of the well.

Thus, there are continuing needs for improved methods of preventing the production of formation sand, fines and the like with produced subterranean formation fluids.

SUMMARY OF THE INVENTION

The current invention provides a permeable cement composition. When fully cured, the permeable cement composition has a permeability of at least 1 darcy. Preferably, the permeability rating of the permeable cement composition is between about 10 darcies and about 50 darcies. Additionally, when fully cured, the permeable cement composition has a uniform compressive strength of at least 345 KPa (50 psi). Preferably, the permeable cement composition has an uniform compressive strength between about 689 KPa (100 psi) to about 13790 KPa (2,000 psi).

The current invention further provides a permeable cement composition comprising a hydraulic cement, graded sand and additional particulate matter. The additional particulate matter is selected from the group consisting of gilsonite, oil soluble particles and mixtures thereof. The cement composition has a permeability of about 1 darcy to about 50 darcies and a uniform compressive strength of about 345 KPa to about 13790 KPa.

The current invention also provides an improved cement slurry suitable for forming a permeable cement sand screen in a downhole environment. The slurry comprises hydraulic cement, graded sand, a diluent, such as water or salt water, and an effective amount of a surfactant. The slurry optionally includes particulates such as but not limited to gilsonite, oil soluble resin, oil soluble polymer fiber, wax and other particles soluble in crude oil. The incorporation of the surfactant enhances the formation of the slurry from components which otherwise would not readily form a slurry.

In another embodiment, the current invention provides an improved cement slurry suitable for use in the downhole environment. For example, the improved cement slurry will readily form a permeable cement sand screen suitable for preventing the production of sand and other particulate matter from an unconsolidated subterranean formation. The improved slurry comprising a diluent, one to three parts by weight hydraulic cement per part diluent, five to twelve parts by weight graded sand per part diluent and an effective amount of a surfactant. Typically, the surfactant will comprise from about 0.01 to about 5 percent by weight of the final cement slurry. The surfactant enables the formation of a slurry from components that would not otherwise readily form a slurry. Additionally, the slurry optionally includes particulates such as but not limited to gilsonite, oil soluble resin, oil soluble polymer fiber, wax and other particles soluble in crude oil.

The current invention also provides a method for forming a permeable cement sand screen downhole. In the method of the current invention the permeable cement sand screen is formed in the portion of the well bore adjacent to a hydrocarbon producing subterranean formation. The method of the current invention comprises forming a slurry comprised of a hydraulic cement, graded sand, a diluent such as water or salt water and an effective amount of a surfactant. Further, the slurry optionally includes particulates such as but not limited to gilsonite, oil soluble resin, oil soluble polymer fiber, wax and other particles soluble in crude oil. The method of the current invention is suitable for practice in cased and uncased wells and may be used in conjunction with currently available production hardware. Typically, the well bore is completed with a casing. The casing and formation are subsequently perforated. Following completion and perforation, the slurry is placed in the annulus between the walls of the well bore and the casing and allowed to set, thereby forming a permeable cement sand screen.

Further, the current invention provides a method for hydraulically fracturing tight rock formations having reservoir permeabilities in the range of less than 1 millidarcies. The method of the current invention comprises completing a well bore by conventional methods including perforating the formation if necessary. Before or after completing the well bore, forming a slurry comprised of a hydraulic cement, graded sand, a diluent such as water or salt water and an effective amount of a surfactant. Additionally, the slurry optionally includes particulates such as but not limited to gilsonite, oil soluble resin, oil soluble polymer fiber, wax and other particles soluble in crude oil. Injecting the slurry downhole to the portion of the formation to be fractured under sufficient hydraulic pressure to fracture the subterranean formation. Subsequently the slurry is forced into the resulting fractures. Once the slurry has penetrated the formation, it is allowed to set within the fractures thereby forming a permeable cement composition within the fractures.

DETAILED DESCRIPTION OF THE CURRENT INVENTION

As noted above, the current invention provides an improved permeable cement composition. The improved permeable cement composition is prepared from a slurry comprising an effective amount of a surfactant. Addition of the surfactant to the relatively dry mixture of hydraulic cement, graded sand and diluent yields a readily pourable slurry. Accordingly, as used herein, the term “an effective amount” refers to the quantity of surfactant necessary to yield at least a pumpable slurry. Preferably, the slurry is pourable following addition of an effective amount of the surfactant.

As shown in the table below, the permeable cement composition of the current invention has high compressive strength and excellent permeability. Preferably, once fully cured the permeable cement composition will have a permeability of at least 1 darcy. More preferably, the permeable cement composition will have a permeability between about 10 and about 50 darcies. Additionally, once fully cured, the improved permeable cement composition will have an uniform compressive strength (UCS) of at least 345 KPa (50 psi). Preferably, the permeable cement composition will have a UCS between about 689 KPa (100 psi) to about 13790 KPa (2,000 psi).

A variety of hydraulic cements can be utilized to form the slurry of the current invention, including but not limited to those comprised of calcium, aluminum, silicon, oxygen and/or sulfur which set and harden by reaction with water. Such hydraulic cements include Portland cements, microfine cements, pozzolana cements, gypsum cements, high aluminum content cements, silica cements and high alkalinity cements. Portland cements or their equivalents are generally preferred for use in accordance with the present invention. Portland cements of the types defined and described in API Specification For Materials And Testing For Well Cements, API Specification 10, 5th Edition, dated Jul. 1, 1990 of the American Petroleum Institute are particularly suitable. Preferred API Portland cements include classes A, B, C, G and H, with API classes G and H being more preferred and class G being the most preferred.

The particulate matter utilized in the performance of the present invention may comprise sand, bauxite, sintered bauxite, ceramic materials, glass beads, foamed ceramics, nut shells, coke, polymer beads or glass materials containing voids produced by gases or other processes such as hollow mineral glass spheres sold under the trade name “SPHERELITE” by Halliburton Energy Services, Duncan, Okla. The types of graded sand used in the slurry of the current invention will determine the permeability and influence the compressive strength of the cured permeable cement composition. In general, graded sand having mesh sizes between 100 mesh and 6 mesh will perform satisfactorily in the current invention. Preferred mesh sizes will range between 60 and 20 mesh.

The diluent used in the slurry provides the moisture necessary to yield a pourable or at least a pumpable slurry when combined with a surfactant. Diluents suitable for use in the current invention include fresh water or salt water. The term “salt water,” as used herein, refers to unsaturated and saturated salt solutions such as but not limited to brines and seawater.

As previously indicated, a surfactant is added to the diluent, hydraulic cement and sand mixture to produce a pourable or at least pumpable slurry of cement. Preferably, the surfactant is an alkyl poly ether having the formula RO(CH₂CH₂O)_(n)R′ where R and R′ are alkyl groups and n is an integer in the range of about 5 to about 50. An alternative surfactant suitable for use in the current invention has the formula: RPhO(OCH₂CH₂)_(m)OH, wherein R is an alkyl group having from about 5 to about 30 carbon atoms, Ph is phenyl and m is an integer in the range of from about 5 to about 50. A preferred surfactant in this group is ethoxylated nonylphenol containing in the range of from about 20 to about 30 moles of ethylene oxide.

Another surfactant appropriate for use in the current invention has the general formula: R₁(OR₂)_(n)SO₃X, wherein R₁ is selected from the group consisting of alkyl groups having from 1 to about 30 carbon atoms, cycloalkyl groups having 5 or 6 carbon atoms, C₁-C₄ alkyl substituted cycloalkyl groups, phenyl, alkyl substituted phenol of the general formula (R₃)_(a)Ph—, wherein Ph is phenyl, R₃ is an alkyl group having from 1 to about 18 carbon atoms and a is an integer of from 1 to 3, and phenyl-alkyl groups wherein the alkyl groups have from 1 to about 18 carbon atoms and the phenyl-alkyl groups have a total of from about 8 to about 28 carbon atoms; R₂ is a substituted ethylene group of the formula —CH₂CH₂R₄, wherein R₄ is selected from hydrogen, methyl, ethyl or mixtures thereof; n is a number from 0 to about 40 provided that when R₁ is phenyl or alkyl substituted phenyl, n is at least 1; and X is any compatible cation.

Another suitable surfactant for use in the current invention is a salt having the general formula: R₅—Ph(OR₆)_(o)SO₃X, wherein R₅ is an alkyl radical having in the range of from 1 to about 9 carbon atoms, R₆ is the group —CH₂CH₂—, o is an integer from about 10 to about 20 and X is a compatible cation. Preferably, the cation is sodium.

Yet another surfactant which can be utilized is a salt having the formula: R₇(OR₈)_(p)SO₃X, wherein R₇ is an alkyl group having in the range of from about 5 to about 20 carbon atoms, R₈ is the group —CH₂CH₂—, p is an integer in the range of from about 10 to about 40 and X is a compatible cation. A preferred surfactant of this type is the sodium salt of a sulfonated compound derived by reacting a C₁₂-C₁₅ alcohol with about 40 moles of ethylene oxide commercially available under the name “AVANEL S150™” or by reacting a C₁₂-C₁₅ alcohol with about 15 moles of ethylene oxide having the formula: H(CH₂)₁₂₋₁₅(CH₂CH₂O)₁₅SO₃Na, (both referred to hereinafter as an “ethoxylated alcohol sulfonate”) commercially available under the name “AVANEL S400™” both compounds are commercially available from PPG Mazer, a division of PPG Industries, Inc. of Gurnee, Ill.

Of the various surfactants described above which can be used, ethoxylated nonylphenol containing in the range of from about 15 to about 40 moles of ethylene oxide and the sodium salt of a sulfonated and ethoxylated compound having the formula H(CH₂)₁₂₋₁₅(CH₂CH₂O)₁₅SO₃Na are preferred, with H(CH₂)₁₂₋₁₅(CH₂CH₂O)₁₅SO₃Na being the most preferred.

The foregoing components and other suitable additives are combined to yield a permeable cement composition slurry which is at least pumpable, more preferably, the slurry is pourable. Any acceptable method for forming a cement slurry may be used to form the slurry of the current invention. In general, the slurry will comprise, based on the weight of the diluent component, one to three parts hydraulic cement, five to twelve parts graded sand and an effective amount of surfactant. Preferably, the surfactant will be added to the liquid diluent in an amount sufficient to produce a slurry of desirable consistency. Typically, the slurry comprises about 0.01 to about 5 percent of the surfactant by weight. Other components suitable for inclusion in the permeable cement of the current invention include particulates soluble in crude oil. For example, particulates such as but not limited to gilsonite, oil soluble resin, oil soluble polymer fiber, wax and other particles soluble in crude oil. During production, oil from the formation passing through the cement will dissolve these particles thereby increasing the permeability of the cement by increasing the void space within the cement. Thus, particles of this nature improve the permeability of the cement without detrimentally impacting the strength of the cement.

Table I below compares eight different formulations of the current invention. As demonstrated in Table I, the slurry of the current invention provides a permeable cement composition having sufficient permeability to permit the production of hydrocarbons through the cured cement into the well bore.

TABLE I Composition Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Sample 7 Sample 8 Brine (2% 30 30 30 30 30 15 15 15 CaCl2, mL) Class G cement, 45 45 45 45 45   22.5   22.5   22.5 (g) Sand (grams) & 210 of 270 of 270 of 210 of 270 of 150 of 165 of 180 of mesh size 40/60 40/60 40/60 20/40 20/40 40/60 40/60 40/60 Surfactant    0.05    0.05    0.05    0.05    0.05    0.08    0.08    0.08 (mL) Stress during  0  0 1724  1724  1724  1724  1724  1724  cure, KPa @ {250} {250} {250} {250} {250} {250} 65.6° C. {psi, @  150° F.} Permeability 14 50 20  9 25 23 30 44 (Darcies) Unconfined 5647  6143  10990   6833  8874  7350  4985  3689  Compressive {819} {891} {1594} {991} {1287} {1066} {723} {535} Strength KPa {psi}

The permeable cement composition of the current invention will be useful in a wide variety of applications. For example, the current invention provides a method for forming a permeable cement sand screen suitable for filtering fluids produced from a subterranean formation. According to this embodiment of the current invention, a well bore penetrating a subterranean formation is completed according to methods known to those skilled in the art. For example, a typical well bore completion entails installing and perforating a casing followed by fracturing the formation. Following perforation of the casing, a slurry of the permeable cement composition of the current invention is injected downhole into the casing and the annulus between the well bore walls and the casing. Once the permeable cement sets, the cement located inside the casing is reamed or drilled out to leave behind a permeable cement sand screen in the annulus. As noted above, once cured, the permeable cement composition of the current invention is sufficiently permeable to permit production of formation fluids through the cured cement composition into the well bore. Preferably, the permeable cement sand screen has a permeability of at least 1 darcy. More preferably, the permeable cement sand screen has a permeability ranging from about 10 darcies to 50 darcies. Thus, the permeable cement secures the casing within the well bore and eliminates the need for gravel packs and other similar sand filtration devices.

Additionally, the current invention provides an improved method for fracturing a subterranean formation. In particular, the permeable cement composition provides the means for fracturing relatively “tight” formations. For the purposes of this disclosure, a “tight” formation is defined as a formation having a permeability of less than 1 millidarcy. The method of the current invention provides for preparation of a permeable cement as described above. Prior to pumping the cement downhole, the well bore is preferably completed with a casing and perforated to provide fluid communication between the interior of the casing and the formation. The permeable cement composition is pumped downhole where it contacts the formation face through the perforations. Sufficient pressure is applied during the pumping process to initiate a fracture at the formation face. Pumping continues to force the permeable cement composition into fracture thereby propagating the fracture outwards into the formation. Following completion of the fracturing process, the permeable cement composition is allowed to cure while sufficient pressure is maintained on the well bore to preclude closure of the fractures.

Other embodiments of the current invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. However, the foregoing specification is considered merely exemplary of the current invention with the true scope and spirit of the invention being indicated by the following claims. 

1. A method for forming a permeable cement sand screen in a well bore adjacent to a fluid producing subterranean formation therein comprising the steps of: preparing a slurry comprising a hydraulic cement, graded sand, a diluent and an effective amount of surfactant; completing the well bore with a casing; perforating the casing and the subterranean formation; and, placing the slurry in the annulus between the perforated casing and the walls of the well bore and allowing the slurry to set therein thereby forming a permeable cement sand screen.
 2. The method of claim 1, wherein the diluent comprises fresh water or salt water.
 3. The method of claim 1, wherein the hydraulic cement is selected from the group consisting of: Portland cements, microfine cements, pozzolana cements, gypsum cements, high aluminum content cements, silica cements, high alkalinity cements and mixtures thereof.
 4. The method of claim 1, wherein the surfactant is selected from the group consisting of surfactants corresponding to the following formulas: RO(CH₂CH₂O)_(n)R′ where R and R′ are alkyl groups and n is an integer in the range of about 5 to about 50; RPhO(OCH₂CH₂)_(m)OH, wherein R is an alkyl group having from about 5 to about 30 carbon atoms, Ph is phenyl and m is an integer in the range of from about 5 to about 50; R₁(OR₂)_(n)SO₃X, wherein R₁ is selected from the group consisting of alkyl groups having from 1 to about 30 carbon atoms, cycloalkyl groups having 5 or 6 carbon atoms, C₁-C₄ alkyl substituted cycloalkyl groups, phenyl, alkyl substituted phenol of the general formula (R₃)₃Ph—, wherein Ph is phenyl, R₃ is an alkyl group having from 1 to about 18 carbon atoms and a is an integer of from 1 to 3, and phenyl-alkyl groups wherein the alkyl groups have from 1 to about 18 carbon atoms and the phenyl-alkyl groups have a total of from about 8 to about 28 carbon atoms; R₂ is a substituted ethylene group of the formula —CH₂CH₂R₄, wherein R₄ is selected from hydrogen, methyl, ethyl or mixtures thereof; n is a number from 0 to about 40 provided that when R₁ is phenyl or alkyl substituted phenyl, n is at least 1; and X is any compatible cation; R₅—Ph(OR₆)_(o)SO₃X, wherein R₅ is an alkyl radical having in the range of from 1 to about 9 carbon atoms, R₆ is the group —CH₂CH₂—, o is an integer from about 10 to about 20 and X is a compatible cation; R₇(OR₈)_(p)SO₃X, wherein R₇ is an alkyl group having in the range of from about 5 to about 20 carbon atoms, R₈ is the group —CH₂CH₂—, p is an integer in the range of from about 10 to about 40 and X is a compatible cation; and, mixtures thereof.
 5. The method of claim 1, further comprising the step of fracturing the producing formation prior to the step of placing the slurry within the annulus between the casing and the well bore walls.
 6. The method of claim 1, further comprising the step of fracturing the producing formation by injecting the slurry under hydraulic pressure sufficient to fracture the producing formation.
 7. The method of claim 6, further comprising the step of maintaining hydraulic pressure within the well bore for a period of time sufficient to allow the slurry to cure thereby forming a permeable cement composition within the resulting fractures. 